The present invention relates to a turbocharger system for combustion engines for example, internal or external combustion engines such as an Ericsson engine, and more particularly to an improved turbocharger system including a compressor driven by a turbine from the exhaust gases and which produces supercharging over a broad range of engine operation, i.e. from idle to full power operation.
Present day turbochargers usually use a turbine driven by an engine exhaust gas, the turbine having a shaft connected to drive a compressor; the compressor operating to increase the pressure of incoming air. The turbocharger may be installed either to suck through a carburetor or to blow through the carburetor or may be used with fuel injection systems.
As is known, one of the problems with present day turbocharges using turbines and compressors is that at low flow range of engine exhaust gases, e.g. the idle mode, there is insufficient flow of exhaust gases to drive the turbine. Conversely, at full power mode, present day systems either provide an abundance of supercharging (with possible engine damage) or flow of exhaust gas to the turbine would be restricted, thus increasing the backpressure of the engine to unacceptable levels, unless suitable control device such as a dumping valve or "waste gate" is used. The present use of such devices results in a large loss of otherwise usable power.
Further, the compression ratio of the engine and the performance of the turbocharger system must be "matched" so to speak so that the turbocharger provides the desired supercharging in the range of engine operation for which it is desired. Presently, the turbocharger is usually designed so that the maximum beneficial effect of turbocharging is achieved at or near the engine's most efficient design range, or an engine is turbocharged to achieve a specific performance, e.g. high initial acceleration, high initial horsepower. In the case of pleasure vehicles, the turbocharger and engine are matched to provide optimum performance in the usual cruising range of the vehicle.
For example, at idle an engine is not at its most efficient operation and the volume of exhaust gases is low in comparison to cruising speed or full power.
Since the boost pressure of the turbocharger increases approximately as the square of the speed of the compressor, the usual practice is to control the speed of the turbine in order to control the speed of the compressor. In this way the proper boost pressure is obtained in that desired range of engine operation. Due to the variation in exhaust gas flow of the engine from the idle to full power operation, it has theretofore been difficult to obtain efficient turbocharger-engine performance over the range of engine operation from idle to full power.
More specifically the performance (speed) of a turbine of a fixed size and geometry is related to the pressure and flow of the inlet fluid. As the pressure and flow increase, the turbine speed (and thus the compressor speed) increases. Moreover, it is known that in the case of turbomachinery, there is a range of optimum performance.
Accordingly, the problem with prior turbocharger systems has been the fact that optimum operation is over a limited range, e.g. fast acceleration, brief bursts of power or cruising range, the reason is that usually the turbocharger system includes some form of a "waste gate" system in the gas flow path from the engine exhaust to the turbine. At the idle condition of the engine, the waste gate is closed and all of the engine exhaust, or a substantial portion, enters the turbine, the flow being at a reduced volume compared to acceleration or cruising mode of engine operation. There is normally very little, if any, supercharging effect, unless the system is designed specifically to operate in the idle through a portion of acceleration range of engine operation.
As the engine accelerates, more exhaust gases at a higher pressure flow into the turbine, causing it to increase in speed and causing the compressor speed to increase, resulting in an increase in boost pressure. Once a steady state condition in reached in which the turbocharger and engine performance are matched, the system operates at its design efficiency. If, however, engine speed is increased further, the excess exhaust gases are dumped through the waste gate in order to assure that the turbomachinery continues to operate at or near its design efficiency, and to prevent an increase in the back-pressure.
The result is that on each side of the design condition, fuel economy is reduced over that at the design condition. The only saving in fuel (or increase in power) is in that range in which the engine and turbocharger are running at their matched design ranges, usually a cruising range for most engines and vehicles.
It is also well-known in the art of turbomachinery that given a turbine or compressor of given design and geometry, the maximum efficiency is over a rather select range of impeller speed, flow, and pressure condition. Thus, from a practical standpoint, it is difficult to provide a turbocharger system which is effective over a range of engine performance because of the variation in the needs of the engine and the available energy to drive the turbine.